To "take the 5th" or "plead the 5th" means to claim protection under the fifth amendment to the U.S. Constitution. This amendment says in part that no one "shall be compelled, in any criminal case, to be a witness against himself." If a person believes that his testimony might tend to provide incriminating evidence against himself, he can invoke the fifth amendment and he will not be required to answer.
Interviewer: Why did you leave your last job?
Interviewee: They accused me of stealing four computers
Interviewer: Did they make you confess?
Interviewee: I took the fifth